Optical study of Spintronics in
III-V semiconductors
Xiaodong Cui
University of Hong Kong
Collaborators
• Spin Dynamics
• Magneto-photocurrent
Dr. Yang Chunlei
Mr. Dai Junfeng
Theorist:
Dr. Lu Hai-Zhou
Prof. Shen Shun-Qing
Prof. Zhang Fu-Chun
Outline
 Time resolved Kerr-rotation spectroscopy in
the Spin dynamics study
 Spin Photocurrent in two dimensional
electron gases of InGaAs
Kerr Rotation spectroscopy
Classical picture:
Change in the polarization state when a linearly
polarized light reflected from a strong magnet.
Magnetization ↔Bound currents boundary conditions
E


B1  B 2  K  nˆ
M
• Microscopic origin – selection rule

mj=-1/2
mj=+1/2
2
1
3
mj=-3/2
-1/2
mj=-1/2
Pump beam: Creating Spin Polarization
via Optical injection.
Probe beam: A linearly polarized light is a
superposition of a left and right circularly
Polarized lights.
+1/2
mj=+3/2
mj=+1/2

M2
M3
Ti:Sapphire
I1
M1
I2
I3
I4
M4
PBS1
M6
PEM
M8
Pump
Sample
Probe
M5
M7
Chopper
Y
A
G
M9
M10
BS1
/2 Plate
f1
f2
LA1
LA2
L3
BS2
L5
M11
PBS2 L4
DET
M: Mirror
I: Iris
DET: Twin detector
PBS: polarized beam splitter
LC: lock-in amplifier
L: lens
g-factor
Existing techniques to study g factor:
 Electric transport
Low temperature, high requirements for sample quality
 Electron spin resonance
unpaired electron
 Magneto-photoluminescence
complex origins, signal reflects information of exciton
Kerr-rotation spectroscopy
Magnitude, NO sign information
z
y
x
 

 
Torque driving precession T    B  (  g  B /  ) S  B
Spin projection along ZS ( t )  S exp(  t /  ) cos( g  Bt /    )
Z
0
S
B
S Z ( t )  S 0 exp(  t /  S ) cos( g  B Bt /    )
(a) GaAs thin film
g=-0.42 (T=5K)
(b) GaAs 2DEG
g=-0.36 (T=5K)
(c) GaAsN/GaAs
quantum well
(N~1.5%)
g=+0.97
GaAsN/GaAs quantum well
Phase shift is determined by the
experimental configuration 
S Z ( t )  S 0 exp(  t /  S ) cos( g  B Bt /    )
For g>0
Phase term gBBt/ħ+ for B>0
gBBt/ħ- for B<0
Another Approach – magnetic field scan at fixed time delay
Magnetic field shift is
determined by the experimenta
configuration 
S 0 exp(  t /  S ) cos( g  B Bt /    )
Advantage against time scan:
• time shift in time scan ~ ps
• magnetic shift in field scan
~ 102-103 Gauss
Electric current and spin current
The electric current
The spin current
J c   e  j  j   0
Js 

2
j

 j   0
Generation of Spin current
 Spin injection
Spin polarized charge current
Non-local spin injection
 Optical injection
Intra-band Linearly polarized light:
Ganichev et al., Nature Physics 2, 609 (2006).
two
Inter-band Linearly polarized light (one photon,
photon):
H. Zhao et al., PHYSICAL REVIEW B 72, 201302 2005; Phys. Rev. Lett.
96, 246601 (2006).
Bhat et al., Phys. Rev. Lett. 85, 5432 (2000).
Spin pumping (ferromagnetic resonance)
 Spin Hall effect
Generation and Detection of Spin current -- Spin Hall effect
Converting to magnetization
Converting to charge current
Valenzuela, S. O. & Tinkham, M. Nature 442, 176–179 (2006).
Awschalom, Science 306, 1910–1913 (2004)
Kimura, Phys. Rev. Lett, 98, 156601 (2007)
Wunderlich; Phys. Rev. Lett. 94, 047204 (2005)
Wunderlich, Nature Physics, 5,675 (2009)
Zero-bias spin separation
Ganichev et al., Nature Physics 2, 609 (2006).
Intra-band excitation with linearly polarized THz radiation
Spin dependent excitation and relaxation process
(001)
C2V symmetry
H=(xky- ykx)
Incident light: 0.8eV Linearly polarized light (Band edge excitation)
Rashba coefficient =4.3E10-12 eVm
J(Bx, By, )= C0By + CxBxsin2 + CyBycos2
(c)
Estimate the spin current
 Measurement of Photocurrent with Hall Effect
J~ 1.5X10-2A/m at 1mW
 Estimate the spin current from SdH oscillation
JS ~ J
n
 4  10
4
A/m
n
 Estimate the ratio of field induced charge current
Vs. zero field spin current
J x ( B ) / J S ( B  0 )  1 . 7  10
2
/ Tesla
The magnetic field induced charge current vs. pure spin current
1 E  (k )
V k 
x

x/ y
sx
k x

(
m
  , k

k
*
1
2
v

x
sin 
2
) cos  
,
x/ y
k
hy
 , k 
 cv , k   cv   cv cos 2 cos 2   cv sin 2 sin 2
0
cos
sin
Magnetic field induced charge current density
Pure Spin photocurrent density(ħ)
The ratio
~
 k
2
2
2m
*
~ h / k

~
k / m
g B B
In our case, Fermi energy ~ 10-1~10 -2eV (n=9E11cm-1),
Zeeman energy hu=1.2E-4 eV/Telsa (g= -0.4)
The Ratio ~ 10-2 ~10-3 /Tesla
*
Conclusion
 Magnetic field induced photocurrent via direct interband transition by a linearly polarized light
 Our experiments support that the spin photocurrent
could be generated by linearly polarized light
absorption in material with spin-orbit coupling.
 The conversion of spin current to magnetic field
induced photocurrent is around 10-2~10-3 per Tesla.